1. Field of the Invention
The present invention relates to a radio communication apparatus and a radio communication method for performing radio communications between a mobile station and a plurality of base stations that constitute communication zones.
2. Description of Related Art
FIG. 6 is a block diagram showing a conventional radio communication apparatus disclosed in Japanese patent application laid-open No. 63-115428/1988, for example. In FIG. 6, the reference numeral 1 designates an antenna of a mobile station; reference numerals 2 and 3 each designate a receiver for receiving data transmitted from a base station; reference numeral 4 designates a switch for connecting one of the receivers 2 and 3 to an output terminal 5; 6 designates an input terminal; 7 designates a transmitter for transmitting data of the mobile station to the base stations; 8 designates a switch for connecting one of synthesizers 9 and 10 to the transmitter 7; 9 designates a synthesizer for establishing a receiving frequency of the receiver 2 and a transmitting frequency of the transmitter 7; 10 designates a synthesizer for establishing a receiving frequency of the receiver 3 and a transmitting frequency of the transmitter 7; and 11 designates a controller for controlling the oscillation frequencies of the synthesizers 9 and 10, and the connections of the switches 4 and 5.
Next, the operation of the conventional communication apparatus will be described.
First, the controller 11 connects the switch 4 to the receiver 2 and the switch 8 to the synthesizer 9 as shown in FIG. 6 to tune the receiving frequency of the receiver 2 at a transmitting frequency of the corresponding base station and the transmitting frequency of the transmitter 7 at a receiving frequency of the base station.
This makes it possible for the mobile station to exchange data between the base station using the receiver 2 and the transmitter 7. However, if the mobile station roams out of the communication zone of the base station during the communications, the data exchange with the base station will become impossible. To avoid such a situation, the mobile station must establish synchronization with a contiguous base station before the data exchange becomes impossible, and perform handoff from the current base station to the contiguous base station.
However, since the handoff is carried out by establishing synchronization with the contiguous base station while maintaining communications with the base station connected through the current link, it will cause a hit (instantaneous interruption) of the voice or data during the communications.
To reduce the duration of the hit of the voice or data to a minimum in the conventional example, the current base station notifies the mobile station of the channel number of the contiguous base station in advance.
When the receiver 2 of the mobile station receives the channel number of the contiguous base station while continuing the communications with the current base station without changing the receiving frequency of the receiver 2 and the transmitting frequency of the transmitter 7, the controller 11 controls the synthesizer 10 to tune the receiving frequency of the receiver 3 at the frequency corresponding to the channel number of the contiguous base station.
This makes it possible for the receiver 3 to receive the data transmitted from the contiguous base station, to measure the receiving level of the data, and thus to establish the synchronization with the contiguous base station.
Subsequently, when the receiver 2 receives a handoff command from the current base station, the controller 11 connects the switch 4 to the receiver 3 and the switch 8 to the synthesizer 10, thereby carrying out switching of the communications from the current base station to the contiguous base station.
With such a configuration, although the conventional radio communication apparatus can reduce the duration of the hit of the voice or data during the handoff to a minimum, it has a problem of increasing the circuit scale and power consumption of the mobile station because it includes both the receiver 2 and the receiver 3 for establishing the synchronization with the contiguous base station.